The US Fish and Wildlife Service (FWS) proposed a plan Monday to protect a weasel-like species known as a fisher from rat poison, which is regularly used on pot farms, according to reports.

Fishers would be listed as a threatened species under the Endangered Species Act in the states of Oregon, California and Washington if the agency's request is approved. A final decision is expected by Sept. 30, 2015 after a period of public comment.

Other reasons for the proposal include the loss of forest habitat to wildfire, logging and urban development, disease, illegal fur trapping and climate change.

"This is a complex and challenging issue because threats to the fisher vary across its range," Robyn Thorson, director of the Service's Pacific Region, said in a statement.

But the most concerning threat posed to these forest-dwelling mammals - part of a family that includes weasels, mink, martens, and otters - is rat poison.

"Rodenticide use has been verified at illegal marijuana cultivation sites within occupied fisher habitat on public, private and tribal lands in California," the FWS said in the statement. "Although the service does not know the full extent to which rodenticide exposure causes injury or mortality of fishers, rodenticide exposure in fishers has been documented in fisher populations in the Klamath Mountains and Southern Sierra Nevada, as well as in the reintroduced population at Olympic National Park in Washington."

Erin Williams, who oversaw the FWS analysis, told The Associated Press that the poisons are regulated, but that the rules have done little to stop their misuse.

According to the agency's proposal, 84 percent of the 77 fisher carcasses found in California's northwestern mountains and the southern Sierra Nevada tested positive for rat poison. What's more, just about all of fisher habitat includes illegal marijuana plantations.

Fishers don't even have to eat the poison directly to feel its fatal effects. Ingesting prey that ate the poison used to kill rats, which might eat marijuana plants, is deadly too. And marijuana cultivation is only likely to rise in Washington state, where recreational use was recently legalized.

This is bad news for these fishers that have "virtually disappeared," according to the FWS, a species currently estimated at less than 4,000.